1. Field of the Invention
The present invention relates to a water-developable, photosensitive resin composition containing a completely or partially saponified polyvinyl acetate having a high image-reproducibility.
2. Description of the Prior Art
Recently, photosensitive resin printing plates for relief printing, lithographic or intaglio processes having a structure comprising a metal or plastic base provided with a photopolymerizable, photosensitive resin layer on the surface thereof have been put to practical use. In using these printing plates, a negative or possitive original film having transparent areas is brought into intimate contact with the photosensitive resin layer and irradiated with actinic rays to effect the photopolymerization in the photosensitive resin layer in areas corresponding to the transparent areas of the original film and the resin in the non-polymerized areas is washed out with a suitable solvent to form a relief image on the base.
These photosensitive resin compositions have been used for various purposes including the preparation of printing plates, taking advantage of their photopolymerizability. Among the photosensitive resin compositions, it has been proposed to use a partially saponified polyvinyl acetate as a base resin, because it allows the resin in the non-polymerized areas to be washed out with neutral water to effect the development. Such compositions are disclosed in, for example, the specifications of U.S. Pat. Nos. 3,630,746, 3,801,328 and 4,042,386. These known compositions comprise a partially saponified polyvinyl acetate and a photopolymerizable monomer such as .beta.-hydroxyethyl methacrylate. To realize an excellent water developability, the partially saponified polyvinyl acetate to be used should have an average degree of polymerization of as low as less than 500. However, when the partially saponified polyvinyl acetate having such a low degree of polymerization is used, the water resistance of photo-hardened areas becomes insufficient and, therefore, no excellent image reproducibility can be obtained easily. Further, since the degree of polymerization of the polymer base is low, cracks are formed in the relief or the relief is abraded seriously in the printing step. Thus, it has been difficult to realize both excellent water developability and excellent image reproducibility and printability in the prior art wherein the partially saponified polyvinyl acetate has been used as the base resin.
Further, a photosensitive resin composition containing a vinyl copolymer containing a dialkylamino group or its salt as the base resin has also been known. For example, the specification of Japanese Patent Laid-Open No. 192420/1982 discloses a photosensitive resin composition comprising a vinyl copolymer obtained by a copolymerization of a vinyl monomer containing a dialkylamino group, a copolymerizable vinyl monomer, a photosensitizer and an organic halogen compound. However, the resulting photosensitive printing plate has a water developability far inferior to that of known printing plates containing the partially saponified polyvinyl acetate, since the vinyl copolymer contained in the former has only a poor water solubility. The specification of Japanese Patent Laid-Open No. 199342/1983 didcloses a photosensitive resin composition comprising a vinyl copolymer (obtained by copolymerizing a hydroxyl group-containing vinyl monomer with a dialkylamino group-containing vinyl monomer) and a diazo compound. However, the water developability of the resulting photosensitive printing plate is also inferior to that of the known printing plates, since the water solubility of the vinyl copolymer is inferior to that of the partially saponified polyvinyl acetate having a low degree of polymerization. In addition, no excellent image reproducibility can be expected from the photosensitivity of the diazo compound. The specification of Japanese Patent Laid-Open No. 21736/1983 discloses a photosensitive resin composition containing as a base polymer a modified, partially saponified polyvinyl acetate having a cationic group including a dialkylamino group. In this polymer, however, the water solubility of the partially saponified polyvinyl acetate itself is reduced, since the absolute amount of the hydroxyl group which makes the partially saponified polyvinyl acetate water-soluble is reduced by the introduction of the cationic group. Accordingly, the development rate of the photosensitive printing plate with water is rather reduced.
As described above, it has been difficult to realize both excellent water developability and high image reproducibility and printability in the known photosensitive resin printing plates comprising the partially saponified polyvinyl acetate as the base polymer or the known photosensitive resin printing plates comprising the dialkylamino group-containing polymer as the base resin.